1. Field of the Invention
This invention relates to a pressure sensor, particularly to such a sensor used under a high pressure condition of, for example, more than 500 kgf/cm.sup.2, and utilizing a semiconductor, for example, a piezoresistance element, as a pressure detecting element. The pressure sensor of this invention can be used, for example, in an oil pressure regulating system for construction equipment and the like.
2. Description of the Related Art
There are many different kinds of known pressure sensors generating an electrical signal corresponding to a pressure detected, each using a different method of detecting a pressure, but a pressure sensor having a semiconductor pressure detecting element in which a piezoresistance element is provided on a silicon substrate is now most widely used.
A pressure sensor provided with such a semiconductor pressure detecting element is known as a liquid-seal type pressure sensor, and is used under a condition such that the pressure detecting element is sealed with a suitable liquid such as a silicon oil 104 or the like, to eliminate a problem of deterioration of the element.
An example of the construction of such a semiconductor pressure detecting element is illustrated in FIG. 7.
In FIG. 7, a pressure detecting element 103, consisting of a piezoresistance element formed on a surface of a silicon substrate, is firmly mounted on the surface of a mount 102 made of an insulating material such as a borosilicate glass.
The mount 102 is hermetically fixed to a housing 100 of the sensor in such a way that the pressure detecting element 103 is inserted in a cavity 101 formed inside the housing 100 and serving as a sensor room S, and a lead wire 107 is provided in the mount 102 in such a manner that it penetrates therethrough and is hermetically sealed to the mount 102 with an insulating sealing material 108 such as a glass or the like, and one end thereof projects into the cavity 101 and serves as a hermetic terminal to enable contact between the lead wire 107 and the detecting element 103 through a suitable wire f, to thereby transmit an electric signal output by the pressure detecting element 103, to the outside. Further, a connecting aperture R for communicating one surface of the detecting element 103 with the atmosphere is provided in the mount 102, and a circumferential portion of a sealing diaphragm 105 covering the detecting element 103 and separating the cavity 101 into a sensor room S, is fixed to the housing 100 by welding or the like.
A passage 120 for communicating the inside portion of the sensor room S with the outside is provided in the housing 100, and a sealing liquid such as a silicon oil or the like is injected therethrough and into the sensor room S, after the detecting element 103 and the sealing diaphragm 105 are mounted on the housing 100, and a vacuum is established therein, and then a sealing element such as plug 121 or the like is inserted into the passage 120.
Finally, a flange portion 113 having an aperture 115 through which the pressure medium enters the cavity to come into contact with the diaphragm is fixed to the housing 100 by a bolt 114 and a hermetic packing such as an O-ring or the like therebetween, to complete the assembly of the pressure sensor.
In the thus-assembled pressure sensor, a pressure medium, such as oil, water, air, and other liquids, is introduced into a pressure medium introducing portion 116 provided in the pressure sensor through the aperture 115 provided in the flange portion 113, whereby the pressure detecting element 103 detects the pressure of the pressure medium, through the sealing diaphragm 105 and the silicon oil sealed inside the sensor room S, and generates an electric signal corresponding to the pressure detected, at the output of the detecting element 103 and this signal is transmitted to the outside through the lead wire 107.
The components of the liquid-seal type pressure sensor are not deteriorated by contact with the pressure medium such as a liquid or the like, and the sensor has superior characteristics in that it is environment proof and has a high level of reliability, due to the sealing of the detecting surface of the pressure detecting element 103 by a stable liquid such as a silicon oil or the like.
Nevertheless, in this kind of pressure sensor, the pressure of the pressure medium to be measured is directly applied to the liquid contained inside the sensor room S, and this pressure may reach a very high level of 2000 atms when the sensor is used in oil pressure equipment.
In this case, when the pressure sensor shown in FIG. 7 is used, the pressure of the pressure medium is also directly applied to the plug 121 through the silicon oil, and since one end surface of the plug 121 is exposed to the atmosphere, and the other end surface thereof is under pressure from the silicon oil 104, a problem may arise in that, if the plug 121 is not firmly fixed in the housing 100, a leakage of the liquid will occur at the plug 121 and thus the reliability of the pressure sensor device will be lowered.
FIG. 8 shows a different type of pressure sensor from that shown in FIG. 7. In the pressure sensor shown in FIG. 8, a housing 100 having a cavity 101 formed at one end thereof in which a mount 102 and a semiconductor pressure detecting element 103 are inserted, is provided. The cavity 101 is filled with a sealing liquid 104 such as a silicon oil or the like, and further, the sealing liquid is hermetically sealed by a sealing diaphragm 105 in the same manner as in the previous sensor. Also, to transmit an electrical signal output through a lead wire f, by the detecting element 103 to the outside, a lead wire 107 is provided in the housing 100 in such a manner that the wire 107 penetrates through the housing 100 and is hermetically sealed to the housing 100 with an insulating sealing material 108 such as a glass or the like; one end thereof projecting into the cavity to form a hermetic terminal in contact with the lead wire f, and the other end thereof projecting into a non-pressurized portion of the housing 100 and in contact with a circuit substrate 109.
Thereafter, the thus-assembled pressure sensor is mounted on a suitable aperture provided in a portion of a device containing a pressure medium therein, by a screw thread 111 having a sealing ring 112 therebetween.
In this case, a high pressure medium applied to the sensor from the direction indicated by an arrow A in FIG. 8, can be sealed at the O-ring 112 made of rubber.
The thus-mounted pressure sensor operates in the same way as the previously described sensor, and has the same effect.
In this kind of pressure sensor, however, the value of the pressure which is applied to the entire pressure sensor by the pressure medium is determined by the diameter of the O-ring 112, i.e., the larger the diameter thereof, the greater the pressure received by the entire pressure sensor.
As shown in FIG. 8, since the pressure sensor of the prior art has a construction such that a screw thread is provided closer to the pressure medium, compared with the position at which the O-ring 112 is located, the diameter of the O-ring 112 must be larger than that of the screw portion.
Therefore, another problem arises in that the reliability of the mechanical strength of the screw thread portion 111 is lowered because the diameter of the O-ring 112 must be restricted, and therefore, when used as a pressure sensor for detecting a high pressure of more than 500 kgf/cm.sup.2, the value of the pressure applied to entire pressure sensor will become very large. Countermeasures intended to solve this problem have not been taken, and the use of a semiconductor pressure sensor for detecting a high pressure of more than 500 kgf/cm.sup.2 and having a high reliability has not been attempted.
Further, in this kind of pressure sensor, another problem arises in that a leakage of the sealing liquid will frequently occur at the hermetically sealed portion 108.
Namely, as shown in FIG. 9, when a high pressure is applied to the surface of the bottom portion of the cavity of the housing 100, especially the portion close to the hermetic terminal, the portion of the housing surrounding the insulating sealing material 108 is expanded outward thereby and the silicon oil leaks out through the contacting surfaces of the housing and the sealing material, or between the sealing material and the lead wire as indicated by an arrow B in FIG. 9, and thus the characteristic of the sensor is deteriorated.
Recent demands for a measurement of a higher pressure oil (or fuel) or the like in the vehicle have led to a need for the use of a semiconductor pressure sensor for such high pressure measurements.